The present invention concerns a valve drive for a valve of an internal combustion engine, wherein the valve is connected to a main piston which is arranged in a cylinder chamber and which for opening of the valve can be acted upon by pressure fluid flowing into the cylinder chamber, and wherein there is provided a closing spring which urges the valve in the closing direction.
In the case of hydraulic valve drives, it is desirable to decelerate the closing movement of the valve, before the valve head comes into contact with the valve seat, in such a way that the speed of contact of the valve with its seat does not become too high. In that case, excessive valve wear would occur. In the known arrangements, shortly before the valve reaches the closed position, a piston engages into a cylinder with a defined, small diameter difference. Due to the narrow gap between the piston and the cylinder, oil flows out of the cylinder through the narrow gap at high speed and with a high pressure difference, whereby the contact movement of the valve is damped. That principle is moreover also used in all kinds of hydraulic shock absorbers to afford a terminal abutment damping action.
The disadvantage of this arrangement is that, for reasons of production tolerance, a relatively large percentage of the valve stroke movement must be employed for the damping procedure in order to afford an adequate damping action. In addition the strength of the damping action and thus the speed of impact of the valve against its seat depends to a high degree on the diameter tolerance of the damper piston and cylinder and thus the cross-section of the gap. In addition the damper travel must be so great that an adequate damping action is produced both in the new condition and also when the valve and the valve seat are worn. The difference in the damper travel between a new and a worn valve/valve seat system may however certainly be up to 5 mm in the case of large engines. Due to the necessity for over-sizing of the damper travel, energy losses which increase with increasing valve wear occur due to excessively strong damping. Furthermore the valve movement becomes increasingly slower in the proximity of the valve seat so that an unwanted variation in the valve closing time can occur.
Therefore the object of the invention is to provide a valve drive which is easier to produce and which is substantially independent of the valve wear condition.
In accordance with the invention that is achieved in that there is provided a throttle by way of which a part of the pressure fluid flows out of the cylinder chamber in a throttled flow during the closing movement of the valve.
Due to the throttle, the speed of the valve as it approaches the valve seat is reduced in such a way that it comes into contact with the valve seat xe2x80x98softlyxe2x80x99.
From the structural point of view it is particularly simple if there is arranged in the cylinder chamber on the side of the main piston remote from the valve an intermediate piston which is movable relative to the main piston and which is provided with a throttle bore. The intermediate piston subdivides the cylinder chamber so that a part of the pressure fluid has to flow away through the throttle bore.
In order to fill the cylinder chamber between the main piston and the intermediate piston in each cycle with an amount of pressure fluid which is adequate for the damping action, it is preferably provided that a compression spring is disposed between the main piston and the intermediate piston. By virtue of the spring which presses against the main piston and the intermediate piston, pressure fluid is sucked into the intermediate chamber when the spring is relieved.
It is desirable if the intermediate piston has a preferably peg-shaped or pin-shaped abutment, on the side which is towards the pressure oil line. The abutment provides that a hollow space is formed on the side of the intermediate piston which is towards the pressure oil line, that hollow space ensuring that the pressure oil can unimpededly flow in and out by way of the pressure oil line independently of the position of the throttle bore.
The size of the throttle bore has an essential influence on the damping action. It has proven desirable for the diameter of the throttle bore to be less than 10% and preferably between about 5 and 7% of the diameter of the intermediate piston.